1) Field of the Invention
The present invention relates to a display apparatus in which brightness of a current-controlled light emitting element is controlled.
2) Description of the Related Art
An organic EL display apparatus in which an organic electroluminescence (EL) element (Organic Light Emitting Diode) is used, has been sought to be used practically as the next generation display apparatus because it is suitable for thinning of the apparatus as it does not require a back light, which is necessary in a liquid crystal display apparatus and there is no limitation on an angle of visibility. Moreover, the organic EL element that is Used in the organic EL display apparatus differs from the liquid crystal display which controls a liquid crystal cell by the voltage in that brightness of each light emitting element is controlled by the current flowing therethrough.
In the organic EL display apparatus, a simple (passive) matrix type and an active matrix type can be adopted as a driving system. The former, though has a simple structure, has a problem of difficulty in realization of a big-size and a highly defined display. For this, in recent years, a development of active matrix type in which a current flowing through a light emitting element inside a pixel, controls an active element that is provided in the pixel at the same time, for example a thin film transistor (TFT), has been carried out actively.
FIG. 20 is a pixel circuit in an organic EL display apparatus of the active matrix type according to a conventional technology. The pixel circuit in the conventional technology, has a structure that includes an organic EL element 105 in which a cathode side is connected to a positive power supply Vdd, a TFT 104 in which a drain electrode is connected to an anode side of the organic EL element 105 and a source electrode is connected to ground, a capacitor 103 that is connected between a gate electrode of the TFT 104 and ground, and a TFT 102 in which a drain electrode in connected to the gate electrode of the TFT 104, a source electrode is connected to a data line 101, and a gate electrode is connected to a scan line 106.
An operation of the pixel circuit mentioned above is described below. When an electric potential of the scan line is allowed to be of a high level and a writing electric potential is applied to the data line 101, the TFT 102 is put ON, the capacitor 103 is either recharged or discharged, and a gate electrode potential of the TFT 104 becomes the writing electric potential. Further, when an electric potential of the scan line 106 is allowed to be of a low level, the TFT 102 is put OFF and the scan line 106 and the TFT 102 are disconnected electrically, however a gate electrode potential of the TFT 104 is maintained to be constant by the capacitor 103.
Then, a current flowing through the TFT 104 and the organic EL element 105 is a value in accordance with a voltage Vgs between the gate and the source of the TFT 104 and the organic EL element 105 continues to emit light having brightness in accordance with this current. Here, the operation of conveying brightness information that is supplied to the data line 101 upon selecting the scan line 106, to an inside of a pixel is called as writing from here onward. As mentioned above, in the pixel circuit shown in FIG. 20, once a potential is written, the organic EL element 105 continues to emit light having a constant brightness (for example, refer to Japanese Patent Application Laid-open Publication No. H8-234683). Here, in the active matrix type organic EL element display apparatus, a TFT formed on a glass substrate is used as an active element.
However, in a TFT that is formed by using amorphous silicon, when current has flown for a long time, there is a problem that a threshold voltage fluctuates from a voltage during the time when the current was flowing. Moreover, there is a problem of a fluctuation in the threshold voltage due to deterioration of the TFT. Thus, the TFT that is formed by using amorphous silicon may cause fluctuation of the threshold voltage in the same pixel.
FIG. 21 is a graph that shows voltage-current characteristics of a TFT before deterioration and a TFT after deterioration. In FIG. 21, a curve l3 indicates characteristics of voltage Vgs between a gate and a source of the TFT before deterioration and drain current Id, and a curve 14 indicates characteristics of the TFT after deterioration. Moreover, Vth4 and Vth4′ are threshold voltages of the TFT before deterioration and after deterioration. As shown in FIG. 21, since the threshold voltages of the TFT before deterioration and after deterioration differ, when the same electric potential VD4 is written, drain currents Id2 and Id3 for each have different values. Therefore, by applying the electric potential VD4, in spite of the fact that only Id2 has flown in the organic EL element before the deterioration of the TFT which is the driver element, no current except Id3 (<Id2) flows after the deterioration of the TFT and light of a predetermined brightness cannot be displayed. Due to this, when a threshold voltage of a TFT that controls current flowing through a current-controlled light emitting element (hereinafter, “current light emitting element”) fluctuates, in spite of the fact that the same electric potential is applied, the current flowing through the current light emitting element fluctuates and as a result, brightness that is displayed on a display section of a display apparatus becomes non-uniform thereby causing the deterioration of the image quality.